1. Field of the Invention
The present invention relates to a planographic printing plate precursor including a specific copolymer-containing intermediate layer and an image forming layer in this order on a support and a method of producing a copolymer used in the intermediate layer. More specifically, the invention relates to a novel planographic printing plate precursor satisfying both high printing durability on an image area and low scumming on a non-image area.
2. Description of the Related Art
Conventionally presensitized plate precursors having a lipophilic photosensitive resin layer formed on a hydrophilic support have been widely used as planographic printing plate precursors. Desirable presensitized plates have been produced commonly by subjecting the image forming layer to mask exposure (surface exposure) through a lithographic film and then dissolving and removing the non-image areas thereof. In recent years, digital technology, by which image information is processed, stored, and outputted electronically by computer, is becoming increasingly popular. Accordingly, various newer image-output methods compatible with digital technology have been commercialized. As a result, there is an urgent need for a “computer to plate (CTP) technology” that allows direct production of printing plates by scanning a high-directivity light such as a laser beam according to digitalized image information without using a lithographic film, and thus there is also a need for planographic printing plate precursors that are compatible with the CTP technology.
As a planographic printing plate precursor compatible with such scanning exposure, a planographic printing plate precursor in which a lipophilic photosensitive resin layer (hereinafter, referred to as a image forming layer) containing a photosensitive compound that may generate an active species such as a radical or Bronsted acid by laser exposure is formed on a hydrophilic support has been proposed and already commercialized. It is possible to obtain negative planographic printing plates by scanning the planographic printing plate precursor with a laser according to digital information, generating an active species and thus causing physical and chemical changes in the image forming layer, insolubilizing the exposed regions, and developing the images thereon.
In particular, a planographic printing plate precursor in which a photopolymerizable image forming layer containing a photopolymerization initiator superior in sensitization speed, an addition-polymerizable, ethylenically unsaturated compound, and a binder polymer soluble in alkaline developing solution, and additionally an oxygen-blocking protective layer as needed, are formed on a hydrophilic support (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-228109), is superior in productivity, easier in developing processing, and superior in resolution and inking property and is therefore a desirable printing plate precursor with superior printing properties.
For further improvement in productivity, i.e., in plate-making speed, a recording material that employs a photopolymerizable composition containing a cyanine dye having a particular structure, an iodonium salt and an addition-polymerizable compound having an ethylenically unsaturated double bond as the image forming layer and that does not require heat treatment after image exposure has been proposed (see, for example, Japanese Patent Application Publication (JP-B) No. 7-103171), however the recording material is susceptible to polymerization inhibition by oxygen in the air during the polymerization reaction and thus had problems that the sensitivity is lower and the strength of the formed image area is insufficient.
To address these problems, a method of forming a protective layer containing a water-soluble polymer on an image forming layer and a method of forming a protective layer containing an inorganic layered compound and a water-soluble polymer are known (see, for example, JP-A No. 11-38633). Presence of the protective layer enables prevention of polymerization inhibition, acceleration of the curing reaction of the image forming layer, and improvement in the strength of the image region.
A positive planographic printing plate precursor for infrared laser for direct plate making wherein a novolac resin or the like is used as an alkali-soluble resin is known. In particular, a positive planographic printing plate precursor of high image quality for infrared laser is disclosed which has a image forming layer wherein a substance that absorbs light to generate heat, various onium salts, and a positive photosensitive compound such as a quinonediazide compound, are added to an alkali-soluble resin having phenolic hydroxyl groups, such as novolac resin (see, for example, JP-A No. 7-285275).
Generally, these planographic printing plate precursors are provided with an intermediate layer (also referred to as an undercoat layer or an adhesion-improving layer) between a support and a image forming layer in order to improve the adhesiveness between the image forming layer and the support or to improve development removability of a light-unexposed region of the image forming layer (see, for example, JP-A No. 2001-272787).
However, when the conventional intermediate layer is used, development removability may be deteriorated during storage for a long time, particularly under high temperature/high humidity conditions, or when the upper layer of the intermediate layer is coated with a image forming layer, the intermediate layer may be dissolved in or swollen with a solvent in the image forming layer, thus sometimes exerting an adverse effect on printing durability and scumming, so there is demand for development of products capable of coping with improvement in request levels for higher printing durability and lower scumming in recent years.
Use of polymer compounds having acid groups in an intermediate layer is known (see, for example, JP-A No. 2005-99113). In JP-A No. 2005-99113, use of a polymer compound having a sulfonic acid or carboxylic acid as an acid group in an intermediate layer is disclosed. When sulfonic acid is contained as an acid group in a side chain, formation of an alkali metal salt, an ammonium salt or a water-soluble amine salt is disclosed. Such compound gives excellent printing durability and scumming resistance in a specific planographic printing plate precursor, however when used in, for example, a highly durable planographic printing plate precursor having a support whose surface roughness (arithmetic mean deviation of the profile) was increased to improve printing durability, the compound sometimes hardly attains sufficient scumming resistance.
In consideration of the problem, an object of the invention is to provide a planographic printing plate precursor excellent both in prevention of scumming on a non-image area and in printing durability attributable to the adhesiveness between an image area and a support.
Another object of the invention is to provide a method of producing a copolymer, which may easily synthesize a copolymer suitable for the planographic printing plate precursor.
As a result of extensive study, the inventors found that an intermediate layer having a specific polymer compound, and a image forming layer, are formed in this order on an aluminum support, thereby enabling achievement of the objects, and the invention was thereby completed.